1. Field of the Invention
This invention relates to light emitting diodes, and in particular to forming ohmic contacts on the nitrogen face polarity surfaces of gallium nitride based light emitting diodes.
2. Description of the Related Art
Light emitting diodes (LED or LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped n-type and p-type layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted from the active layer and from all surfaces of the LED.
For typical LEDs it is desirable to operate at the highest light emission efficiency, and one way emission efficiency can be measured is by the emission intensity in relation to the input power, or lumens per watt. Each of the oppositely doped layers in a typical LED have a contact layers that is used for applying a bias to the LEDs, and for both it is desirable to provide a low resistivity ohmic contact to reduce ohmic losses during operation of the LED. By reducing or minimizing ohmic losses at the contacts, the efficiency (or lumens per watt) can be improved.
LEDs can be formed of many different semiconductor materials, with recent interest and developments focusing on Group III-nitride based devices, such as gallium nitride (GaN) based LEDs. Devices from this material system can be fabricated using different processes such as metal organic chemical vapor deposition (MOCVD). GaN nitride devices are typically formed on growth substrate, and when using MOCVD alternating gallium (Ga) and nitrogen (N) face polarities form the GaN material on the substrate. The GaN epitaxial layer, including the n-type and p-type layers, typically terminate with Ga-face, so that the top surface of both these layers has a Ga-face polarity.
Ohmic contacts to Ga-face polarity GaN layers have been developed and most commonly comprise Ti and Al based contacts [S. Ruminov et al., Appl. Phys. Lett. 69, 1556 (1996); B. P. Luther et al., Appl. Phys. Lett. 71, 3859 (1997); B. P. Luther et al., J. Electron Mater. 27, 196 (1997)]. Formation of contacts to N- and Ga-face n-GaN using a common Ti/Al metal scheme followed by thermal treatment has also been explored [Joon Seop Kwak et al. Apply. Phys. Lett. 79, 3254 (2001)]. Considerable disparity in electrical behavior was observed between metal contacts to N and Ga-face n-type GaN. While contacts to Ga-face were ohmic when annealed at 500° C. or higher, no improvement was observed for the N-face n-GaN. This anomaly was attributed to polarization effects in III-Nitrides.
Other studies have also reported the disparity between contacts to N-face and Ga-face GaN [Ho Won Jang et al., Appl. Phys. Lett. 80, 3955 (2002); O. Ambacher et al., Appl. Phys. Lett. 85, 3222 (1999)]. Both reports conclude that thermal treatment improves contacts to Ga-face GaN while no effect or improvement was seen on N-face n-GaN. Above 600 C, low resistivity contact was achieved for Ga-face, while N-face n-GaN samples exhibited non-linear or Schottky characteristics at same annealing conditions.
As discussed in Ambacher et al., low resistivity ohmic contact to Ga-face n-GaN is achieved at high temperature due to formation of near-interface AlN and thereby creating degenerate GaN below the AlN. Also polarization-induced 2DEG is formed at AlN and GaN interface causing increase in downward band bending as shown in FIG. 1 (from Ambacher et al.). This reduces the Schottky barrier height to allow for electron tunneling, leading to low contact resistivity on Ga-face samples. By comparison, the N-face typically experiences spontaneous polarization in opposite direction as that of Ga-face, such that an AlN on GaN would support formation of 2DHG with opposite band bending, thereby increasing the Schottky barrier height and discouraging electron tunneling.